Non-contact ultrasonic tonometer

ABSTRACT

A non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee&#39;s eye, in non-contact manner by use of an ultrasonic wave comprises: a probe including a vibrator for making the ultrasonic wave incident on the examinee&#39;s eye and a sensor for detecting the ultrasonic wave reflected from the examinee&#39;s eye; and an observation optical system for observing an anterior segment of the eye, wherein the probe is placed in an optical path of the observation optical system, and the observation optical system forms an image of the anterior segment through a surrounding region of the probe.

TECHNICAL FIELD

The non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee's eye in non-contact manner by ultrasound.

BACKGROUND ART

There is proposed a contact intraocular pressure examination apparatus for measuring intraocular pressure by pressing a distal end of a probe pen against an eye, the probe pen holding a probe device including a vibrator for introducing vibration into the eye and a vibration detecting sensor for detecting the vibration reflected by the eye (see Patent Literature 1: JP2004-267299A).

Also proposed is a non-contact intraocular pressure measurement system for measuring intraocular pressure by making an ultrasonic wave enter in an eye (actually, an eyeball model) and detecting the ultrasonic wave reflected from the eye by use of a sensor (see Non-patent Literature 1: “Development of a new non-contact intraocular pressure measurement system using a phase shift method”, Masayuki JINDE and other three persons, Conference of Institute of Electrical Engineers, Sensors and Micromachines Division, Document p. 93-96, 2007). This system is arranged to measure a phase shift of a reflected wave with respect to a transmission wave as a frequency change, and determine a correlation between an amount of the frequency change and hardness of the eye model.

In the case of the apparatus configuration of Patent Literature 1, however, the probe pen is brought into contact with the eye to measure intraocular pressure and thus a large burden would be given to the eye. The apparatus configuration of Non-patent Literature 1 is merely intended to measure the eyeball model, which is insufficient to measure human eyes. In the case of measuring human eyes, which exhibit involuntary eye movement and visual line movement, the ultrasonic wave characteristics (e.g., frequency and phase) detected by the sensor are likely to vary due to misalignment of the apparatus with the eye, leading to variations in measurement results.

SUMMARY OF INVENTION Technical Problem

The present invention has a purpose to provide a non-contact ultrasonic tonometer capable of easily making alignment of the tonometer with respect to an examinee's eye.

Solution to Problem

To achieve the above purpose, the present invention provides a non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee's eye in non-contact manner by use of an ultrasonic wave, the tonometer comprising: a probe including a vibrator for making the nultrasonic wave incident on the examinee's eye and a sensor for detecting the ultrasonic wave reflected from the examinee's eye; and an observation optical system for observing an anterior segment of the eye, wherein the probe is placed in an optical path of the observation optical system, and the observation optical system forms an image of the anterior segment through a surrounding region of the probe.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective external view of a non-contact ultrasonic tonometer of a preferred embodiment of the present invention;

FIG. 2 is a perspective configuration view of a measurement system and an optical system of the tonometer;

FIG. 3 is a perspective configuration view (partly a block diagram) of a control system of the tonometer;

FIGS. 4A and 4B are views showing examples of an observation screen displayed on a monitor;

FIG. 5 is a view showing a case where an objective lens is placed behind a probe;

FIG. 6 is a view showing a modified example of a fixation target projection optical system and a first alignment mark projection optical system;

FIG. 7 is a view showing a case where the probe is displaced in a direction perpendicular to an optical axis of an observation optical system;

FIG. 8 is a view showing a case where the probe is placed out of an optical path of the observation optical system; and

FIG. 9 is a view showing a case where the tonometer is provided with an eye refractive power measurement optical system.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the present invention will be described below with reference to accompanying drawings. FIG. 1 is a perspective external view of a non-contact ultrasonic tonometer 100 of this embodiment.

The tonometer 100 is a so-called stationary apparatus including a base 1, a head support unit 2 attached to the base 1, a movable unit 3 movably placed on the base 1, and a measurement part 4 that is movably provided on the movable base 3 and contains a measurement system and optical systems mentioned later. The measurement part 4 is moved in a right-and-left direction (an X-direction), an up-and-down direction (a Y-direction), and a back-and-force direction (a working distance direction; a Z-direction) relative to an examinee's eye E by a movement part 6 provided in the movable unit 3. The movable unit 3 is moved in the X- and Z-directions on the base 1 by inclining operation of a joystick 5. The measurement part 4 is moved in the Y-direction by the movement part 6 by a rotating operation of a knob 5 a. The joystick 5 is provided, at its top, with a measurement start switch 5 b. On the movable base 3, a monitor 72 is provided.

FIG. 2 is a perspective configuration view of the measurement system and optical systems of the tonometer 100, showing a case where a probe is placed in an optical path of an observation optical system. FIG. 3 is a perspective configuration view (partly a block diagram) of a control system of the tonometer 100.

A probe (a transducer) 10 placed in front of the examinee's eye E has a vibrator (an ultrasonic wave transmitting section) 11 for making an ultrasonic wave (an incident wave, a transmission wave) incident on the eye E and a sensor (an ultrasonic wave receiving section) 13 for detecting an ultrasonic wave (a reflected wave, a received wave) reflected by the eye E. The probe 10 is constituted of for example two piezoelectric elements arranged one on another. One of the piezoelectric elements is used as the vibrator 11 and the other is used as the sensor 13. In this embodiment, a pulse wave is used as the ultrasonic wave made incident on the eye E but a continuous wave may be used instead.

On the side of the probe 10 closer to the eye E, an acoustic lens 16 is placed to converge the ultrasonic wave from the vibrator 11. This lens 16 comes into focus on the eye E when the probe 10 is placed in proper alignment with the eye E.

The probe 10 is connected sequentially to an amplifier 81, a frequency component analysis section 82, a frequency phase difference specifying section 83, and an arithmetic and control section 70. An electric signal corresponding to an incident wave and a reflected wave is amplified to an appropriate signal level by the amplifier 81 and subjected to frequency component analysis by the analysis section 82 to obtain a spectral distribution of the phase difference with respect to the frequency. The specifying section 83 compares the spectral distribution of the incident wave and the spectral distribution of the reflected wave to specify a phase difference θx which is a difference in phase between the incident wave and the reflected wave at respective frequencies fx. The phase difference θx at the frequency fx will vary according to intraocular pressure (strictly speaking, changes in hardness of a cornea of the examinee's eye E resulting from changes in the intraocular pressure). Accordingly, the arithmetic and control section 70 detects the phase difference θx based on an output signal of the specifying section 83 and obtains the intraocular pressure of the eye E based on the detection result. This method is referred to U.S. Pat. No. 6,854,331 (JP2002-272743A).

The probe 10 and the lens 16 are formed, at respective centers, with an aperture 18 (e.g., a circular hole having a diameter of about 1 mm) through which fixation target projecting light (hereinafter, referred to as “fixation target light”) from a light source 32 and alignment mark projecting light (hereinafter, referred to as “alignment mark light”) from a light source 42 are allowed to pass.

The vibrator 11 and the sensor 13 are electrically connected to a circuit system (the amplifier 81, the analysis section 82, the specifying section 83, the arithmetic and control section 70, and others) disposed out of an optical path of the observation optical system 20 (hereinafter, referred to as an “observation optical path”) with a wiring cable 95. This cable 95 is covered with a cover 96 applied with a coating for absorbing reflected light (e.g., infrared light) from an anterior segment of the eye E. This makes it possible to prevent the reflected light by the anterior segment from diffusing on the surface of the cable 95 to be detected as noise light by a two-dimensional image pickup device 26.

Provided as the optical systems of the tonometer 100 are the observation optical system 20 for observing the anterior segment of the eye E, a fixation target projection optical system 30 for causing the eye E to hold fixation, a first alignment mark projection optical system 40 for projecting an alignment mark in the X- and Y-directions to the eye E, a second alignment mark projection optical system 50 for projecting an alignment mark in the Z-direction to the eye E, and an alignment mark detection optical system 55 for detecting the Z-direction alignment mark projected onto the eye E.

The observation optical system 20, having the optical path in which the probe 10 is placed, forms an image of the anterior segment through a region surrounding the probe 10. Specifically, the observation optical system 20 includes, an objective lens 22, an imaging lens 24, a filter 25, and the image pickup device 26 and provides an optical axis (hereinafter, an “observation optical axis”) L1 in which the probe 10 is placed. Thus, when the observation optical axis L1 is aligned to a predetermined portion (for example, a corneal center or a pupil center of the eye E), the probe 10 is placed in front of the eye E. Furthermore, in the configuration of FIG. 2, the probe 10 is arranged so that the central axis (an extension of the central axis) of the probe 10 is coaxial with the observation optical axis L1. Accordingly, when the observation optical axis L1 is aligned with the predetermined portion of the eye E, the central axis of the probe 10 comes to coincide with the predetermined portion of the eye E and therefore an ultrasonic wave reflected by the eye E can be efficiently detected.

Light sources 38 which emit infrared light to illuminate the anterior segment of the eye E are disposed diagonally to the front of the eye E. The filter 25 has a property of transmitting the light from each light source 38 and the light from the light source 42 and blocking light from a the light source 51.

The light from each light source 38 is projected onto the anterior segment of the eye E and then the light reflected by the anterior segment travels toward the lens 22. The light reaching the surrounding region of the probe 10 passes through the lens 22, further passes a half mirror 36 and a dichroic mirror 46, and forms an image on the image pickup device 26 by the lens 24. Specifically, the anterior segment image by the light source 38 is formed on the image pickup device 26 through the surrounding region of the probe 10. The dichroic mirror 46 has a property of transmitting the light from each light source 38 and the light from the light source 42 and reflecting the light from the light source 32.

A signal output from the image pickup device 26 is input to the arithmetic and control section 70. The picked-up anterior segment image is displayed on the monitor 72. In the configuration of FIG. 2, an imaging optical system (a lens system constituted of a plurality of lenses) including the lenses 22 and 24 is used as a light delivery member for delivering the anterior segment image to the image pickup device 26. Alternatively, a single lens may be used to deliver the anterior segment image to the image pickup device 26.

For a positional relationship between the probe 10 and the lens 22 in the direction of the observation optical axis L1 (the Z-direction), the lens 22 is placed behind the probe 10 (closer to the image pickup device 26) or in the same position as the probe 10 and hence wider space can be provided between the eye E and a housing of the apparatus (see FIGS. 2 and 5). In this case, the reflected light from the anterior segment may be interrupted by the probe 10 and the lens 16. In order to allow clear observation of the anterior segment image, the probe 10 and the lens 22 are preferably placed close to each other on the observation optical axis L1 and more preferably placed in substantially the same position on the observation optical axis L1.

FIG. 2 shows one example of the configuration in which the probe 10 and the lens 22 are placed in the same position. That is, the lens 22 is formed, in almost the center thereof, with an aperture 22 a in which the probe 10 is inserted. In this aperture 22 a, the probe 10 and the lens 16 are set.

FIG. 5 shows one example of the configuration in which the lens 22 is placed behind the probe 10. Specifically, the lens 22 is formed, in almost the center thereof, with the aperture 22 a in which a support member 19 is inserted. The probe 10 and the lens 16 are set in a forward position in the support member 19 inserted in the aperture 22 a.

Alternatively, the lens 22 may be placed before the probe 10. In this case, the lens 22 is preferably formed with an aperture in a position corresponding to an ultrasonic wave propagation path in order to prevent the lens 22 from reflecting and attenuating the ultrasonic wave from the probe 10. Thus, the incident wave from the vibrator 11 is allowed to enter the eye E through the aperture and the reflected wave from the eye E is detected by the sensor 13 through the aperture.

The fixation target projection optical system 30 includes at least a fixation target projection light source to project a fixation target onto the eye E to cause the eye E to view the fixation target. Specifically, the projection optical system 30 includes the light source 32, a fixation target 33, a diaphragm 34, a projection lens 35, and the dichroic mirror 46 to project the fixation target onto the eye E through the aperture 18. An optical axis L2 of the projection optical system 30 is made coaxial with the observation optical axis L1 by the dichroic mirror 46 located in the observation optical path.

Light of the fixation target 33 illuminated by visible light from the light source 32 is reduced in light diameter by the diaphragm 34, passes through the lens 35, is reflected by the dichroic mirror 46, passes through the half mirror 36, and then is projected onto the fundus of the eye E through the aperture 18. Thus, the eye E holds fixation.

The first alignment mark projection optical system 40 includes at least an alignment mark projecting light source to project an alignment mark in the X- and Y-directions onto the eye E from front. Specifically, the projection optical system 40 includes the light source 42, a projection lens 44, and the half mirror 36 to project the alignment marks (the alignment mark light) onto the eye E through the aperture 18. An optical axis L3 of the projection optical system 40 is made coaxial with the observation optical axis L1 by the half mirror 36 located in the observation optical path.

Infrared light from the light source 42 passes through the lens 44, is reflected by the half mirror 36, and then is projected onto the cornea of the eye E through the aperture 18. Light mirror-reflected by the cornea forms an image (an alignment mark image) i1 which is a virtual image (a corneal reflection image) of the light source 42.

The light of the mark image i1 travels toward the lens 22. The light reaching the surrounding region of the probe 10 passes through the lens 22, the half mirror 36, and the dichroic mirror 46, and forms an image on the image pickup device 26 by the lens 24. In other word, the mark image i1 by the light source 42 is formed on the image pickup device 26 through the surrounding region of the probe 10. When the eye E moves in the X- and Y-directions, an image forming position of the mark image i1 also moves on the image pickup device 26. Based on a detection signal of the image pickup device 26, the arithmetic and control section 70 can detect an alignment state of the apparatus (the probe 10) in the X- and Y-directions with respect to the eye E.

In the case of projecting the alignment mark light through the aperture 18 formed in the probe 10 and the lens 16 and receiving the reflected light from the eye E through the surrounding region of the probe 10, part of the reflected light may be interrupted by the lens 16. To avoid such defect, as shown in FIG. 2, the projection optical system 40 is preferably configured as an optical system whereby the alignment mark light is converged once before reaching the eye E and then is projected as dispersion light onto the cornea.

The second alignment mark projection optical system 50 includes at least an alignment mark projecting light source to project an alignment mark in the Z-direction onto the eye E from an oblique direction. Specifically, the projection optical system 50 includes the light source 51 and a projection lens 52 to project the alignment mark (the alignment mark light) onto the eye E. An optical axis L4 of the projection optical system 50 intersects with the observation optical axis L1 at a predetermined angle.

Infrared light from the light source 51 passes through the lens 52, is substantially collimated, and then is projected onto the cornea of the eye E. The light mirror-reflected by the cornea forms an image (an alignment mark image) i2 which is a virtual image (a corneal reflection image) of the light source 51.

The alignment mark detection optical system 55 includes a photo-receiving lens 56, a filter 57, and a position sensitive device 58 (e.g., a line CCD) to detect the alignment mark image formed by the projection optical system 50. The filter 57 has a property of transmitting the light from the light source 51 and blocking the light from the light source 38 and the light from the light source 42. An optical axis L5 of the detection optical system 55 is symmetrical to the optical axis L4 of the projection optical system 50 with respect to the observation optical axis L1. The optical axis L5 intersects with the optical axis L4 at a point on the optical axis L1.

The mark image i2 by the light source 51 is formed on the position sensitive device 58 by the lens 56. When the eye E moves in the Z-direction, an image forming position of the mark image i2 moves on the position sensitive device 58. Based on a detection signal of the position sensitive device 58, the arithmetic and control section 70 can detect an alignment state of the apparatus (the probe 10) in the Z-direction with respect to the eye.

The arithmetic and control section 70 is coupled to the knob 5 a, the switch 5 b, the movement part 6, the monitor 72, the specifying section 83, the light sources 32, 38, 42, and 51, the image pickup device 26, the position sensitive device 58, an operation section (an input section) 74 provided with various switches, a memory 75 serving as a storage section, and others. The arithmetic and control section 70 performs control of the entire apparatus, calculation of measured values, and so on.

The memory 75 stores a table showing a correlation between the phase difference θx at the frequency fx and an intraocular pressure value. The arithmetic and control section 70 retrieves an intraocular pressure value corresponding to the detected phase difference θx from the memory 75 based on the output signal of the specifying section 83 and displays the retrieved intraocular pressure value on the monitor 72.

The correlation between the phase difference θx and the intraocular pressure value can be set by experimentally determining in advance a correlation between phase differences θx obtained by the present apparatus and intraocular pressure values measured by a Goldmann tonometer. The memory 75 stores a program for measuring intraocular pressure by use of the probe 10, a program for controlling the entire apparatus, and so on.

On the operation part 74, there are arranged a selection switch 74 a for selecting either an automatic alignment mode of automatically aligning the measurement part 4 with respect to the eye E or a manual alignment mode of manually aligning the measurement part 4 with respect to the eye E, a selection switch 74 b for selecting either an automatic shot mode of automatically generating a trigger signal to start measurement upon completion of alignment or a manual shot mode of generating a trigger signal to start measurement based on an operation signal of the switch 5 b, and others. When the automatic shot mode is selected, the arithmetic and control section 70 determines whether the alignment state is proper or not based on each detection signal of the image pickup device 26 and the position sensitive device 58. Based on the determination result, the arithmetic and control section 70 generates a measurement-start trigger signal and, based on the generation of the trigger signal, causes the probe 10 to emit an ultrasonic wave to the eye E.

Operations of the apparatus having the above configuration are explained below. Firstly, the face (head) of an examinee is fixed on the head support unit 2. An examiner makes alignment of the apparatus with the examinee's eye E by manipulating the joystick 5 while viewing the monitor 72. At that time, the arithmetic and control section 70 displays the anterior segment image picked up by the image pickup device 26 and a reticle LT and an indicator G for alignment on the monitor 72 as shown in FIGS. 4A and 4B.

When the mark image i1 starts to appear on the monitor 72 (when the image pickup device 26 starts to detect the mark image i1), the automatic alignment in the X- and Y-directions is enabled. Furthermore, when the position sensitive device 58 starts to detect the mark image i2, the automatic alignment in the Z-direction is enabled. The arithmetic and control section 70 controls display of the indicator G based on information about the alignment state in the Z-direction obtained from the detection signal of the position sensitive device 58.

The case of selecting the automatic alignment mode and the automatic shot mode is explained below. The arithmetic and control section 70 obtains misalignment amounts of the apparatus in the X-, Y-, and Z-directions relative to the eye E located in a proper position, and controls driving of the movement part 6 to bring each misalignment amount into a predetermined permissible range. When each misalignment amount falls within the permissible range, the arithmetic and control section 70 stops the driving of the movement part 6 and automatically generates the measurement-start trigger signal to start intraocular pressure measurement.

The case of selecting the manual alignment mode and the manual shot mode is explained below. In this case, the examiner manipulates the joystick 5 (the knob 6 a) so that the mark image i1 displayed on the monitor 72 enters in the reticle LT and the indicator G appears in the form representing alignment completion (see FIG. 4B). When the alignment is completed in each direction and the switch 5 b is pressed by the examiner, the arithmetic and control section 70 generates the measurement-start trigger signal to start the intraocular pressure measurement.

Upon generation of the measurement-start trigger signal, the arithmetic and control section 70 causes the vibrator 11 to emit the ultrasonic wave to the eye E and detects the ultrasonic wave reflected from the eye E by the sensor 13. The arithmetic and control section 70 calculates an intraocular pressure value of the eye E based on the output signal of the specifying section 83 and displays a result thereof on the monitor 82.

With the above configuration, the alignment between the eye E and the probe 10 can be easily performed.

In the above explanation, the fixation target light is projected onto the eye. E through the aperture 18 formed in the center of the probe 10. It is not limited thereto but may be arranged to project the fixation target light through the surrounding region of the probe 10 in the lens 22. A conceivable configuration in this case is, for instance, to use a diaphragm having an annular aperture centered on the optical axis L2 instead of the diaphragm 34 having a spot aperture on the optical axis L2.

In the above embodiment, the alignment mark light is projected onto the eye E through the aperture 18 formed in the center of the probe 10. It is not limited thereto but may be arranged to project the alignment mark light through the surrounding region of the probe 10 in the lens 22, and allow the image pickup device 26 to detect the reflected light passing through the surrounding region of the probe 10 in the lens 22. A conceivable configuration in this case is, for instance, to use an annular light source instead of the spot light source 42.

FIG. 6 is a view showing a modified example of the fixation target projection optical system and the first alignment mark projection optical system. In this case, the light source 32 (e.g., an LED) is placed in the center of the probe 10.

A first alignment mark projection optical system 140 for projecting alignment mark light at a predetermined angle to the observation optical axis L1 is placed outside the lens 22. Reflected light thereof is allowed to pass through the surrounding region of the probe 10 in the lens 22. In this case, the angle of the optical axis of the projection optical system 140 to the observation optical axis L1 is determined to prevent part of the reflected light from becoming interrupted by the lens 16.

In the case where the light source 32 is placed in the center of the probe 10 as shown in FIG. 6, a light source that emits visible light and infrared light may be used as the light source 32 to serve both as the fixation target projecting light source and the alignment mark projecting light source.

In the above explanation, the probe 10 is placed on the observation optical axis L1 but not limited thereto. The probe 10 may be displaced from the observation optical axis L1 in a direction (the X- and Y-directions) perpendicular to the optical axis L1 as shown in FIG. 7. In this case, a detection position of the mark image i1 on the image pickup device 26 when the central axis (the extension of the central axis) of the probe 10 comes into alignment with the predetermined portion (e.g., the corneal center or the pupil center) of the eye E is set as an alignment reference position, and a display position of the reticle LT, an alignment completion position, and others are set.

FIG. 8 is a schematic configuration view of the measurement system and the optical system of the tonometer 100, showing the case where the probe is placed out of the optical path of the observation optical system.

An ultrasonic wave reflecting member (an acoustic mirror) 90 reflects an incident wave from the vibrator 11 toward the eye E while reflecting a reflected wave from the eye E toward the sensor 13. The observation optical system 20 is arranged so that the probe 10 is placed out of the optical path thereof and the observation optical axis L1 is positioned on an ultrasonic wave propagation path between the reflecting member 90 and the eye E. The lens 22 is formed with an aperture 22 b through which an ultrasonic wave from the probe 10 is allowed to pass. The incident wave from the vibrator 11 is reflected by the reflecting member 90 to enter the eye E after passing through the aperture 22 b. The reflected wave from the eye E passes through the aperture 22 b, is reflected by the reflecting member 90, and then is detected by the sensor 13.

In the case where the lens 22 is placed between the reflecting member 90 and the eye E, the lens 22 formed with the aperture 22 b in a portion corresponding to the ultrasonic wave propagation path can avoid attenuation of the ultrasonic wave which is likely to be caused in passing through the lens 22. In this case, the reflecting member 90 applied with a coating having a property of blocking the reflection light from the anterior segment by the light source 38 may be used to prevent the anterior segment reflection light from entering the image pickup device 26 through the aperture 22 b, thereby preventing resultant noise light.

As the reflecting member 90, a member having a property of reflecting the ultrasonic wave and transmitting light (for example, a transparent and colorless, hard plastic plate) may be used. This can prevent the fixation target light and the alignment mark light from becoming interrupted even when the reflecting member 90 is placed in each optical path of the projection optical system 30 and the projection optical system 40. In the case of using the reflecting member 90 having a light transmission property, taking into consideration that the optical length is changed by passage of light through the reflecting member 90, a member having the area almost equal to an optical path splitting member such as the half mirror 36 and the dichroic mirror 46 may be used.

The present invention is not limited to the above configuration and may be arranged such that the reflecting member 90 is partly provided with an aperture through which the fixation target light and the alignment mark light are allowed to pass to be projected onto the eye E. The above configuration shows the case in which the reflecting member 90 is placed in a common optical path of the projection optical systems 30 and 40. The above configuration can be applied to the case where the reflecting member 90 is placed in the optical path of at least one of the projection optical systems 30 and 40.

The configuration in which the probe 10 is placed out of the observation optical path is not limited to one shown in FIG. 8 and may be arranged so that the reflecting member 90 is placed between the lens 22 and the eye E. In this case, the incident wave from the vibrator 11 is reflected by the reflecting member 90 to enter the eye E, while the reflected wave from the eye E is reflected by the reflecting member 90 and detected by the sensor 13.

The tonometer may be additionally provided with a measurement optical system for measuring eye characteristics different from the intraocular pressure. FIG. 9 is a view showing the case where an eye refractive power measurement optical system is added to the tonometer.

An eye refractive power measurement optical system 310 is arranged so that the probe 10 is placed out of an optical path of the measurement optical system 310 (a measurement optical path), and an optical axis L6 of the measurement optical system 310 (hereinafter, referred to as a “measurement optical axis”) is located on the ultrasonic wave propagation path between the reflecting member 90 and the eye E. The reflecting member 90 is placed in front of the eye E. The incident wave from the vibrator 11 is reflected by the reflecting member 90 to enter the eye E and the reflected wave from the eye E is reflected by the reflecting member 90 and detected by the sensor 13. Thus, the intraocular pressure of the eye E is measured.

The measurement optical system 310 is placed on the transmission side of a dichroic mirror 301 located at the rear of the reflecting member 90. The measurement optical system 310 is an optical system for projecting measurement light to the fundus of the eye E and receiving reflected light from the fundus by a photo-receiving device. Based on an output signal of the photo-receiving device, the eye refractive power is measured. The measurement optical system 310 and a measurement principle of eye refractive power are well known and thus their details are omitted herein.

On the reflection side of the dichroic mirror 301, an objective lens 311, a dichroic mirror 312, and a total reflection mirror 313 are placed. On the reflection side of the mirror 313, a fixation target projection optical system not shown is arranged to cause the eye E to view the fixation target.

On the reflection side of the dichroic mirror 312, arranged is an observation optical system 322 including an imaging lens 320 and a two-dimensional image pickup device 321 placed in a substantially conjugate relationship with the vicinity of the anterior segment of the eye E. The image pickup device 321 picks up the anterior segment image formed by a light source 325 and a mark image formed by the alignment mark projection optical system not shown.

The measurement optical axis L6 and an optical axis L7 of the observation optical system 322 are made coaxial by the dichroic mirror 301. The dichroic mirror 301 has a property of transmitting light from a light source of the measurement optical system 310 and reflecting the light from the light source 325, light from a light source of the alignment mark projection optical system, and light from a light source of the fixation target projection optical system. The dichroic mirror 312 also has a property of transmitting the light from the light source of the fixation target projection optical system and reflecting the light from the light source 325 and the light from the light source of the alignment mark projection optical system. Used as the reflecting member 90 is a member having a property of reflecting an ultrasonic wave and transmitting light (e.g., a transparent and colorless, hard plastic plate). This member transmits the measurement light by the measurement optical system 310, the anterior segment reflected light by the light source 325, the fixation target light by the fixation target projection optical system, the alignment mark light by the alignment mark projection optical system, and others.

In the configuration of FIG. 9, the reflecting member 90 is placed in a common optical path of the measurement optical system 310, the observation optical system 322, and the fixation target projection optical system but it is not limited thereto. The configuration has only to reflect the ultrasonic wave from the probe 10 by the reflecting member 90 to enter the eye E from front. For instance, the reflecting member 90 may be placed between the dichroic mirror 301 and the lens 311.

Although the above explanation exemplifies the eye refractive power measurement optical system, the present invention is not limited thereto and may be applied to a measurement optical system for measuring eye characteristics different from intraocular pressure by receiving reflected light resulting from measurement light projected onto the eye E. For example, a non-contact type eye axial length measurement optical system (e.g., see U.S. Pat. No. 7,434,932 (JP2007-37984A), a corneal thickness measurement optical system (e.g., see JP63-197433 (1988)A), and others may be adopted.

Furthermore, a mode of performing intraocular pressure measurement based on the measurement-start trigger signal may be selected as needed with a switch or the like between a normal measurement mode of performing one intraocular pressure measurement in response to one trigger signal and a continuous measurement mode of repeating intraocular pressure measurements several times in response to one trigger signal.

The case of selecting the continuous measurement mode is explained below. Upon generation of the measurement-start trigger signal, the arithmetic and control section 70 causes the probe 10 to continuously emit an ultrasonic wave pulse toward the eye E to obtain information on variations in intraocular pressure caused by pulsation of the eye E, and performs the arithmetic processing corresponding to each ultrasonic wave pulse continuously emitted.

Specifically, the ultrasonic wave pulse is continuously made incident on the eye E at predetermined time intervals (e.g., 0.1 seconds intervals) within a range (e.g., within 1.5 seconds) of a pulsation cycle of the eye E, and an intraocular pressure value corresponding to each ultrasonic wave pulse is calculated. In this way, many intraocular pressure values can be obtained within the range of the pulsation cycle and thus variations in intraocular pressure values in the pulsation cycle can be captured. In this case, based on each measured value obtained within the range of the pulsation cycle, it is possible to calculate a representative value (e.g., an average value of the measured values, a central value of the measured values) and calculate measured values at a peak, a bottom, and a middle of the pulsation.

In the above explanation, the ultrasonic wave pulse is emitted at the predetermined time intervals (e.g., 0.1 seconds intervals) but not limited thereto. The ultrasonic wave pulse may be emitted at a predetermined number of times previously set within the range of the pulsation cycle. The time intervals and number of emissions for continuously emitting the ultrasonic wave pulse may be made arbitrarily settable and a switch thereof may be provided in the operation part 74.

In the above explanation, the alignment state of the apparatus with respect to the eye E in the Z-direction is optically detected (a working distance is detected) but it may be detected by the probe 10 used for intraocular pressure measurement. In this case, the control of the probe 10 has to be switched between control for measuring the intraocular pressure and control for detecting the working distance. In the case of detecting the working distance with respect to the eye E by use of the probe 10, the arithmetic and control section 70 measures a measurement time T from emission of the incident wave from the vibrator 11 toward the eye E until the reflected wave from the eye E is detected by the sensor 13 and thereby detects the working distance of the probe 10 from the eye E. In other words, as the measurement time T from the emission of the ultrasonic wave from the vibrator 11 until the ultrasonic wave is detected by the sensor 13 is longer, the working distance is larger. As the measurement time T is shorter, the working distance is smaller. The arithmetic and control section 70 previously determines a reference measurement time Tk for which the working distance is proper with respect to the eye E and considers the alignment in the Z-direction to be completed when the measurement time T reaches the reference measurement time Tk.

In the case of the above configuration, for example, the arithmetic and control section 70 controls the probe 10 as a working distance sensor with respect to the eye E before completion of alignment and controls the probe 10 as an intraocular pressure measurement sensor with respect to the eye E after completion of alignment. This can facilitate the configuration for detecting the alignment state of the apparatus relative to the eye E in the Z-direction.

The above explanation is made to determine the intraocular pressure based on a difference in acoustic impedance resulting from the phase difference between an input phase and an output phase. The present invention is not limited thereto and may be applied to a configuration that can determine intraocular pressure by performing a comparison and arithmetic processing of the incident wave from the vibrator 11 and the reflected wave detected by the sensor 13. For instance, it may be arranged to determine intraocular pressure by performing a comparison and arithmetic processing of the frequency of the incident wave from the vibrator 11 and the frequency of the reflected wave detected by the sensor 13. Specifically, a phase shift circuit may be provided to shift the phase difference to zero by changing the frequency of an ultrasonic wave generated by the vibrator 11 when the phase difference occurs between the input waveform to the vibrator 11 and the output waveform from the sensor 13. The intraocular pressure is determined by detecting a frequency change amount when the phase difference is shifted to zero. 

1. A non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee's eye in non-contact manner by use of an ultrasonic wave, the tonometer comprising: a probe including a vibrator for making the ultrasonic wave incident on the examinee's eye and a sensor for detecting the ultrasonic wave reflected from the examinee's eye; and an observation optical system for observing an anterior segment of the eye, wherein the probe is placed in an optical path of the observation optical system, and the observation optical system forms an image of the anterior segment through a surrounding region of the probe.
 2. The tonometer according to claim 1, wherein the probe is placed on an optical axis of the observation optical system.
 3. The tonometer according to claim 2, wherein the probe is placed so that a central axis thereof is coaxial with the optical axis of the observation optical system.
 4. The tonometer according to claim 1, wherein the observation optical system includes an image pickup device and an objective lens for forming the anterior segment image on the image pickup device, and the objective lens has an aperture in which the probe is inserted.
 5. The tonometer according to claim 1, wherein the observation optical system includes an image pickup device and an objective lens for forming the anterior segment image on the image pickup device, and the objective lens is placed before the probe and has an aperture through which the ultrasonic wave is allowed to pass.
 6. The tonometer according to claim 1 further comprising a fixation target projection optical system provided with a light source for fixation target projection to project a fixation target onto the eye to cause the eye to view the fixation target, wherein the probe has an aperture through which fixation target light from the light source is allowed to pass.
 7. The tonometer according to claim 1 further comprising an alignment mark projection optical system provided with a light source for alignment mark projection to project an alignment mark onto the eye, and an alignment mark detection optical system provided with an image pickup device to detect a corneal reflection image formed by the light source, wherein the observation optical system forms the corneal reflection image on the image pickup device through the surrounding region of the probe.
 8. The tonometer according to claim 7, wherein the probe has an aperture through which alignment mark light from the light source is allowed to pass.
 9. A non-contact ultrasonic tonometer for measuring intraocular pressure of an examinee's eye in non-contact manner by use of an ultrasonic wave, the tonometer comprising: a probe including a vibrator for making the ultrasonic wave incident onto the examinee's eye and a sensor for detecting the ultrasonic wave reflected from the examinee's eye; an ultrasonic wave reflecting member for reflecting the incident wave from the vibrator toward the eye and reflecting the reflected wave from the eye toward the sensor; and an observation optical system (20) for observing an anterior segment of the eye, wherein the probe is placed out of an optical path of the observation optical system, and the observation optical system is placed so that an optical axis thereof is positioned on an ultrasonic wave propagation path between the eye and the reflecting member.
 10. The tonometer according to claim 9 further comprising a measurement optical system for measuring eye characteristics of the eye different from the intraocular pressure, wherein the probe is placed out of an optical path of the measurement optical system, and the measurement optical system is placed so that an optical axis thereof is positioned on the ultrasonic wave propagation path between the eye and the reflecting member.
 11. The tonometer according to claim 9, wherein the reflecting member has a property of reflecting the ultrasonic wave and transmitting light. 